A. Field of the Invention
The present invention relates to a method of well completion, and more particularly to a method of cementing one or more conduits in a well. The completed well structure is also part of the invention.
B. Description of the Prior Art
In conventional well completion methods, casing is cemented in a wellbore with cementitious material filling substantially the entire annulus between the casing and the borehole. One or more additional concentric conduits, e.g., tubing, may also be cemented inside the casing in a similar manner. It is also known to complete wells where, for various purposes, only a portion of the overlap between tubing and casing members are cemented. An example of such a completion technique for a totally different purpose than that of the present invention is where an insulating fluid is maintained in a portion of an annulus in the completion of wells passing through permafrost.
The present invention is more directly concerned with the completion of wells where it may be desirable to abandon the well within the useful life of the tubular goods, i.e., casing, tubing, liners, or the like, used to complete the well, or where intermediate casing must be run during drilling of the well but may later be removed when the liner is in place. The cost of such an abandoned well or completion procedure is significantly reduced if a substantial proportion of the tubular goods can be recovered from the well for subsequent reuse.
A known technique for completing such wells is to employ a gel, sometimes referred to as a casing gel, in lieu of cementitious material over a substantial length of the tubular good. If abandonment of the well or removal of that string of tubular good later becomes desirable, the tubular good can be severed above the cementitious material using known techniques, and the upper portion of the tubular good adjacent the casing gel can then be retrieved from the well. Such a technique is discussed at pages 79-81 of the June, 1975 issue of Petroleum Engineer.
With the recent increase in deep well wildcatting in the central west Texas area where an intermediate casing string must be set, use of such a technique is desirable. However, prior art casing gels have been based on crosslinked or uncrosslinked organic gelling agents such as galactomannan gums or their derivatives or cellulose derivatives. Such prior art casing gels have been relatively susceptible to thermal and bacterial degradation. Consequently, operators have been reluctant to use the prior art casing gels where drilling is to continue for a considerable time after the intermediate string is set or where the tubular good in question is to removed only if the well is abandoned. Premature degradation of the gel could permit sloughing of intermediate water sensitive zones.